[init] xbmc: don't overwrite an existing exit status file.

Doing so breaks e.g. a GUI-initiated shutdown or reboot.
The XBMC-Upstart bridge add-on saves the exit code to /run/lock/xbmc.quit and
*only after* that it calls "sudo stop xbmc", where the status code file gets
overwritten, always resulting in a application exit (0).